The present invention relates to water-tube boilers such as once-through boilers, natural circulation water-tube boilers and forced circulation water-tube boilers.
The water-tube boiler includes body of which is made up by water tubes. The body arrangement of such a water-tube boiler includes, for example, a plurality of water tubes arranged into an annular shape. In this type of water-tube boiler, a cylindrical space surrounded by the annular water tube array is used as a combustion chamber. In such a water-tube boiler, heat transfer is performed primarily by radiation within the combustion chamber, and heat transfer (primarily by convection) is performed downstream from the combustion chamber. In recent years, such water-tube boilers are also desirable to reduce NO.sub.x and CO emissions.